Plasmon-photon coupled optical devices

ABSTRACT

The present invention is directed to optical devices. More specifically, the disclosed devices include a film defining a periodic array of surface elements so as to give rise to surface plasmon polaritons. The film also includes at least a single aperture having a diameter less than the wavelength of light. In one embodiment, the surface elements can be an array of anisotropic apertures and the films can act as a polarizer. The disclosed devices can also include a material having a variable refractive index substantially adjacent to the metal film. For example, the refractive index of the adjacent material can vary according to some characteristic of the light incident to the device, for instance, the intensity or the angle of incidence of the light. In this embodiment, resonant coupling of incident light with the SPP, and hence transmittivity of the device, can depend upon the nature of incident light. The disclosed devices can be useful in, for example, remote polarizers, polarization mode dispersion, isolators, multi-color displays, switches, such as can be controlled according to incident sunlight, or optical filters, such as for eye protection devices, filtering out possibly harmful light.

BACKGROUND OF THE INVENTION

Over the last century, mankind has developed a growing understanding ofthe nature of light. This growing understanding has led to an increasingability to harness and control light, which has in turn led toimprovements in a wide variety of different technologies. For instance,the ability to control photons has led to improvements incommunications, such as through the development of fiber optics;improvements in opto-electronics, such as through the development ofphoto-voltaic cells; as well as the development of near-field optics, afield of study dedicated to the utilization of near-field light, whichis the light created around the periphery of an object emitting or beingilluminated by light. The study of near-field light has brought aboutthe development of and continuing improvements to many optical devicesincluding many different types of imaging devices as well as opticalscanners, filters, switches, modulators, and the like.

For many years, the low transmittivity of light waves through extremelysmall diameter holes, smaller than the wavelength of the incident light,was a limiting factor to the further development of near-field opticaldevices. Specifically, the transmission of light of a wavelength λthrough an aperture of diameter d, where d<λ, was found to beproportional to (d/λ)⁴. (See for example published U.S. patentapplication 2003/0185135 to Fujikata, et al.) Recently, however, it hasbeen discovered that the level of optical transmission through suchholes can be strongly enhanced through the formation and utilization ofa surface plasmon polariton band structure on the surface of the metalfilm containing the apertures. Specifically, the formation on a metalfilm of a periodic array of sub-light wavelength surface structuralelements, e.g., holes, dimples, or concentric rings, can give rise tothe formation of a well-defined series of surface plasmon polaritons onthe film. Surface plasmon polaritons (also referred to throughout thisdisclosure as simply plasmons or SPP) exist when light couples withsurface plasmons, which are collective electronic excitations at theinterface of a metal (or metallic material) with an adjacent dielectricmaterial. The SPP can be resonantly excited by the impingement ofincident light of a particular wavelength. Specifically, the resonantcoupling between the incident light and the plasmons exist atwavelengths that have been shown to be dependent upon the geometry ofthe periodic array formed on the material, the angle of incidence of theincident light, as well as the refractive index of the dielectricmaterial adjacent to the metal film. The resonantly excited plasmons canpropagate through the apertures in the metal film to the other side and,since their wave nature represents acceleration in electronic charge,they can subsequently reradiate the impinged light.

One of the net results of the resonant coupling of plasmons withincident light can be induced transparencies in nominally opaquematerials that support a plasmon resonance (i.e., permittivity, ε, beingless than zero) when light of the correct wavelength strikes thematerials. In addition, because the photon/plasmon interactions are sostrong, the effects of the resonant coupling effect can be highlyefficient. For instance, these structures can exhibit greater than 100%transmission when compared to the transmission expected from the totalarea defined by the holes.

Research is continuing in an effort to expand the application of thesedevices. For instance, Kim, et al. (U.S. Pat. No. 6,040,936), which isincorporated herein by reference, disclose an optical transmissionmodulation apparatus including a metal film having a periodic array ofsub-light wavelength-diameter holes and a supporting layer including amaterial displaying a selectively variable refractive index. Thematerial displaying a selectively variable refractive index can be, forexample, a liquid crystal material, a ferro-electric liquid crystal, asemiconductor layer, or a polymer electro-optic film, i.e., materials inwhich the refractive index can be controlled through application of anelectric field to the material.

Though such advances represent great improvement in the art, thereremains room for variation and further improvement in the field.

SUMMARY OF THE INVENTION

In one embodiment, the presently disclosed invention is directed to anoptical device that includes a film defining at least one aperture thathas a diameter less than the wavelength of light. In addition, the filmincludes a periodic array of structural elements that can enable theexcitation of an SPP on the film with the allowable energy and momentumthat can provide for coupling of photons and plasmons.

Substantially adjacent to the film, the devices can include a firstlayer, for instance a substrate or a coating layer, that includes amaterial having a selectively variable index of refraction. Morespecifically, the refractive index of the first layer adjacent to thefilm can vary according to some characteristic of the light incident tothe device. For example, the refractive index of the first layer canvary depending upon the intensity of incident light or angle ofincidence of incident light.

The film of the optical device can be formed of any material that cangive rise to a series of surface plasmons. For example, the film can bea metal or a heavily doped semi-conductor. In one embodiment, the filmcan be formed of silver, aluminum, chromium, gold, titanium, or an alloythereof.

In one embodiment, the film can include a single aperture fortransmission of light and the periodic array giving rise to the plasmonscan be a periodic array of surface structures such as dimples,concentric rings, or the like. In another embodiment, the film caninclude an array of apertures for transmission of light that are alsothe surface structures that can give rise to the plasmons. Individualapertures in the film (whether in a film including a single aperture oran array thereof) can generally have a diameter of between about 50 andabout 500 nanometers for visible to NIR light applications.

The periodicity of the structures giving rise to the SPP can vary asdesired. For example, in one embodiment, the periodic array of surfaceelements can have a periodicity of between about 1.414·λ_(p) and about 2μm, wherein λ_(p) is the plasma frequency, so as to give rise toplasmons of multiple modes.

The layer substantially adjacent to the film can include a nonlinearoptical material that can exhibit a variable refractive index dependingupon a characteristic of the incident light. For example, in someembodiments, the material can be one with nonlinear refractive indexgreater than about 10⁻¹⁶ cm²/W. Suitable materials can include, but arenot limited to: arsenic, sulfur, selenium, or germanium-containingchalcogenide glasses; silicon, germanium, or lead-containing oxideglasses; silicon, germanium, zinc, sulfur, selenium, cadmium, lead, ortellurium-containing semiconducting crystals; or nonlinearchromophore-containing polymers.

The devices of the disclosed invention can optionally include additionallayers. For example, additional layers can be included on either side ofthe devices that can, for instance, sandwich the first layer between thefilm and an additional layer or optionally sandwich the film between thefirst layer and an additional layer. In one embodiment, the device canbe a multi-layer device with additional layers on both sides of thedevice. Additional layers can be any suitable material. For example, inone embodiment, an additional layer can include a material having avariable index of refraction including, for instance, a variable indexof refraction depending upon a characteristic of the incident light,similar to the first layer material, or an electro-optic material, inwhich the index of refraction can vary with regard to an electric fieldestablished across the material.

Devices of the disclosed invention can include, for example, opticalswitches, optical limiters, optical filters, and optical modulators. Inparticular, during use, light can be incident upon the devices.Depending upon the characteristics of the incident light, the index ofrefraction of the material forming the substantially adjacent firstlayer can vary. The wavelength of the incident light can then be alteredto a second wavelength in accord with the index of refraction of thesubstantially adjacent first layer, and this light, at the secondwavelength, can couple with the SPP of the film and can be emitted fromthe device.

In one particular embodiment, the disclosed devices can beself-regulating optical filters in which the incident light to thedevice can resonantly couple with the surface plasmons at apredetermined intensity. In this manner, the device can act as a filterto prevent transmission of light at intensities away from that which canresonantly couple with the device. For example, the device can bedesigned so that only low intensity light can couple with the plasmonsand transmittivity of high intensity light is limited. For instance, inone embodiment, light with an intensity between about 10² and about 10¹⁵photons of green light per square centimeter per second can resonantlycouple with the plasmons and exhibit enhanced transmittivity through thedevice, while transmission of higher intensity light can be limited orprevented.

In another embodiment, the present invention is directed to opticaldevices including a film as described above and included on the film canbe two different periodic arrays of apertures that overlay each other.Both of the arrays can be formed of apertures in which the maximumcross-sectional dimension of the aperture is less than the wavelength oflight. At least one of the arrays can be formed of high aspect ratioapertures, that is, apertures having differing length and width suchthat the ratio of length to width is greater than one. In oneembodiment, the high aspect ratio apertures can have an aspect ratiogreater than about two.

According to this embodiment, the second array of apertures can beformed of either anisotropic apertures or apertures of unity aspectratio, as desired. For example, in one embodiment, both arrays can beformed of anisotropic apertures, and the major axes of the apertures ofthe two arrays can be normal to each other on the film.

Films of the present invention that include an array of anisotropicapertures, that is, apertures having an aspect ratio greater than one,can be utilized to modulate light in unique ways. For example, in oneembodiment, a film including a periodic array of anisotropic aperturescan be used to modulate polarized light, for instance polarized lightcritical to applications in display and optical fiber communicationstechnologies. In this embodiment, polarized light can be incident to thefilm. In one embodiment, when the incident polarized light has anelectric field that is parallel to the major axis of the anisotropicapertures, the incident light can be prevented from transmitting throughthe film. In the opposite case, when the incident polarized light has anelectric field that is normal to the major axis of the anisotropicapertures, the incident light can be reradiated from the oppositesurface of the film. Thus, in this embodiment, the disclosed devices canbe utilized to modulate polarized light.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of this invention, including the bestmode to one of ordinary skill in the art, is set forth in thisspecification including the following Figures in which:

FIG. 1 illustrates one embodiment of the disclosed invention including aperforated film adjacent to a dielectric substrate layer and showingincident light striking the device from the side of the device includingthe dielectric substrate layer;

FIG. 2 illustrates the embodiment shown in FIG. 1, with the incidentlight striking the device from the side of the device including theperforated film;

FIG. 3 illustrates another embodiment of the disclosed inventionincluding a perforated film adjacent a supporting substrate layer and anadditional auxiliary layer adjacent the supporting substrate layer;

FIG. 4 illustrates another embodiment of the disclosed inventionincluding a perforated film sandwiched between two supporting substratelayers;

FIG. 5 illustrates a perforated film including a periodic array of highaspect ratio apertures; and

FIGS. 6A-6C illustrate the arrays of a perforated film including twooverlaying periodic arrays, both arrays being formed of high aspectratio apertures.

Repeat use of reference characters in the present specification anddrawings is intended to represent the same or analogous features orelements of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to various embodiments of theinvention, one or more examples of which are set forth below. Eachembodiment is provided by way of explanation of the invention, notlimitation of the invention. In fact, it will be apparent to thoseskilled in the art that various modifications and variations may be madein the present invention without departing from the scope or spirit ofthe invention. For instance, features illustrated or described as partof one embodiment, may be used in another embodiment to yield a stillfurther embodiment. Thus, it is intended that the present inventioncover such modifications and variations as come within the scope of theappended claims and their equivalents.

In one embodiment, the present invention is directed to optical devicesin which the nature of light transmitted through the device can bedependent upon the nature of the incident light, i.e., self-regulatedtransmissive devices. More specifically, the disclosed devices can beplasmon-enhanced optically transmissive devices in which thetransmissive characteristics of the device can depend upon certaincharacteristics of the incident light. For instance, in one embodiment,the intensity of the incident light can effectively determine thetransmittivity of the device. In another example, the angle of incidenceof the incident light can determine the transmittivity of the device.

For purposes of this disclosure, transmittivity is herein defined to bethe transmission efficiency of light at a particular wavelength througha device perforated with sub-light wavelength diameter hole(s).

In another embodiment, the invention is directed to films including aperiodic array of sub-light wavelength anisotropic, high aspect ratioapertures such that the films are polarization sensitive. In thisparticular embodiment, the films can transmit incident light that is ofa particular alignment with the perforated film. As such, the device canact as a light polarizer or a polarization converter.

In one embodiment, the invention is directed to optical devicesincluding two different periodic arrays that overlay one another on asingle film, wherein at least one array is formed of anisotropicapertures. In this embodiment, the devices can be utilized to, forinstance, transmit multiple colors from a single region of the device.

Utilization of the disclosed optical devices can provide self-regulatedas well as highly efficient and fast optical devices such as opticalswitches, modulators, filters, and limiters based on the incident lightintensity, polarization, or an externally applied electric field. Forexample, in one embodiment, the disclosed devices can be employed as awavelength selective filter, wherein only light in a relatively narrowrange of incident intensity can be transmitted by the device. Suchdevices can be utilized in, for example, optical sensors or eyeprotection applications, polarization maintaining devices, and/orpolarizers for displays. Similarly, the disclosed devices can beutilized in self-regulated optical switching or modulating applications.For instance, the disclosed devices can be utilized in remote switchingor modulating applications wherein the device is self-regulated basedupon the incident light to the device.

The optical devices of the disclosed invention include a film that hasbeen etched, milled, or otherwise shaped to include a periodic array ofsurface elements that can give rise to the formation of surface plasmonson the film when the film is impinged by light. In addition, the filmcan include at least one aperture having a diameter less than thewavelength of the incident light. The film is formed of a material thatis capable of giving rise to plasmons, for example the film can be ametallic film.

In one embodiment, substantially adjacent to this film, the discloseddevices can also include a dielectric layer through which light canpass. As the light passes through the adjacent layer, the wavelength ofthe light can be variably altered depending upon the nature of theincoming light. More specifically, the layer can be formed of orotherwise include a material that has a refractive index that dependsupon a characteristic of the incoming light. As such, the transmittivityof the incident light through the device can be self-regulated and candepend upon the nature of the incoming light.

Referring to FIG. 1, a portion of an optical device generally 10according to the present invention is illustrated. As can be seen,device 10 can include a film 11 that has been formed to include an arrayof circular apertures 12 of a diameter d set at a periodic spacing p.Film 11 can generally be a metal film, though in certain embodiments,film 11 can be formed of other suitable materials such as, for example,a heavily-doped semi-conductor material. A non-limiting exemplary listof materials suitable for forming film 11 can include silver, aluminum,chromium, gold, titanium, or alloys thereof.

As the basic nature of the transmission of light through a sub-lightwavelength perforated film is generally known in the art, a detaileddescription of the phenomenon will not be included here. Suffice it tosay that the transmittivity of light at a given wavelength can dependupon the coupling of the light at that wavelength with SPP, with maximumtransmission occurring at resonant coupling. Thus, the transmissionpeaks depend upon the characteristics of the SPP formed which can dependupon both the geometry of the individual surface elements as well as theperiodicity of the array formed, which together give rise to theplasmons. As such, physical characteristics of the system, includingdiameter d, periodicity p, and thickness t of the film 11 can be variedas is generally known in the art in order to form a device that cancouple with light of a specific wavelength. In general, however, thediameter or maximum width (in the case of non-circular apertures) ofapertures 12 can be less than the wavelength of the maximum wavelength λof interest. For example, where a well-defined transmission peak is ofinterest, each aperture can have a diameter on the order of about 0.1λ.In those embodiments wherein large transmission is of interest, theapertures can be larger, for example, on the order of 0.5λ. For example,in one embodiment, d can be between about 50 and about 500 nanometersfor transmission of visible to NIR light.

The thickness t of film 11 can be large enough such that the film isopaque to light, however, thickness t of film 11 can vary over arelatively wide range. For instance, thickness t can be between about 4and about 40 times the optical skin depth of the particular materialforming film 11. In one embodiment, thickness t of film 11 can be about5 times the optical skin depth of the material. For purposes of thisdisclosure, the optical skin depth of a material is herein defined asthat penetration distance where the electric field intensity is (1/e) ofthe incident electric field intensity.

In the embodiment illustrated in FIG. 1, film 11 defines an array ofcircular apertures 12 at a periodic distance p from one another. Whetherformed of apertures, as in FIG. 1, or formed of purely superficialelements, such as surface dimples or concentric rings, the periodicity pfor any array of the disclosed invention can generally be between about300 nm and about 2000 nm. In one embodiment, the periodicity can bebetween about 450 and about 1000 nm.

Apertures 12 can be formed according to any process as is generallyknown in the art. For instance, in one embodiment, the film 11 can beperforated utilizing a focused ion beam, for example a focused ion beamusing gallium ions. Alternative perforation methods can be utilized,however, including electron-beam fabrication methods, holographicembossing technology, or self-assembly of polymer spheres at the surfaceof a substrate that can then be used as a mask pattern or template forfabricating the holes, as described by C. Haginoya, et al.,“Nanostructure array fabrication with a size-controllable naturallithography,” Applied Physics Letters, Vol. 71, No. 20, pp. 2934-2936(Nov.17, 1997), which is incorporated herein by reference thereto.

It should be understood that the perforated film of the disclosedinvention is not limited to the particular embodiment illustrated inFIG. 1. For example, in other embodiments, the film can includeapertures of a different cross-sectional shape, including, for instance,apertures which vary in cross-sectional shape or size across the depthof the film as well as variations in shape of the apertures themselves,e.g. squares, rectangles, ellipses, etc. as opposed to the illustratedcircular apertures, particular embodiments of which will be discussedbelow.

Optionally, the film can include only a single aperture for lighttransmission in combination with a series of superficial surfaceelements such as dimples or concentric rings that give rise to theformation of the plasmons on the film. In this embodiment, the surfaceelements can be formed according to any suitable process known in theart, e.g., etching, milling, deposition of the metal onto structureddielectrics, and the like. Moreover, the array of surface elements,whether superficial or penetrating elements, can be any periodic arrayas is generally known in the art, including, for example, rectangulararrays, triangular arrays, and the like.

Referring again to FIG. 1, device 10 includes a layer 20 substantiallyadjacent to film 11. For purposes of this disclosure, the term‘substantially adjacent’ is herein defined as being either in directcontact with or within an effective distance of the metal film 11. Morespecifically, any distance between the metal film 11 and the layer 20should be less than the distance at which the plasmons of the metal film11 can no longer interact with the layer 20, i.e., less than thedistance at which the e-field intensity falls to 1/e for the dielectricmaterial in the intervening space. Material in the intervening space maybe any dielectric material including a vacuum, air, or some otherdielectric material, each of which would have a characteristic allowabledepth. The depth of this intervening material need be small enough toassure that plasmons arising on film 11 can still interact with thelayer 20. For example, in one embodiment, the metal film 11 can be at adistance from the layer 20 of less than about 200 nm, though thepreferred distance can obviously vary depending upon the particularmaterials of the device. It should be understood, however, that thisparticular exemplary range is in no way intended to limit the scope ofthe invention.

Layer 20 can, in one embodiment, be formed exclusively of a dielectricmaterial with a variable refractive index. In another embodiment, layer20 can include such a material in its make-up without necessarily beingformed exclusively of that material.

The index of refraction of the layer 20 is variable. Moreover, thisvariation can depend upon one or more characteristics of the incominglight. For example, in one particular embodiment, layer 20 can be formedfrom a nonlinear optical material in which the index of refraction ofthe layer 20 can vary depending upon the intensity of the incominglight.

In one embodiment, the material can be one possessing large second andthird order susceptibilities. A non-limiting list of possible materialsfor layer 20 can include arsenic (As), sulfur (S), selenium (Se), orgermanium (Ge)-containing chalcogenide glasses, silicon (Si), germanium(Ge), or lead (Pb)-containing oxide glasses, silicon (Si), germanium(Ge), zinc (Zn), sulfur (S), selenium (Se), cadmium (Cd), lead (Pb), ortellurium (Te)-containing semi-conducting crystals, or nonlinearchromophore-containing polymers.

Other possible materials for layer 20 can include second-order nonlinearcrystals such as β-barium borate, gallium arsenide, gallium phosphide,silicon carbide, indium arsenide, indium antimonide, indium galliumarsenide, indium gallium phosphide, indium gallium arsenide phosphide,lead niobate, lead titanate, lead zirconate titanate, lithium niobate,lithium borate, potassium dihydrogen arsenatem potassium dihydrogenphosphate, potassium titanyl arsenate, potassium titanyl phosphate (aswell as cesium and rubidium analogs), strontium titanate, zinc oxide,zinc sulphide, zinc selenide, lead sulphide. Organic second and thirdorder nonlinear materials also are considered for these devices withspecific exemplary materials including those disclosed in R. Sutherland,“Handbook of Nonlinear Optics,” (Marcel Dekkar, New York, 1996) and theHandbook of Photonics, M. Gupta, ed., (CRC Press, Boca Raton, Fla.,1997), which are incorporated herein by reference.

The thickness of layer 20 can vary widely. For instance, in oneembodiment, layer 20 can be very thin, such as a film or a coatingformed on metal film 11. In another embodiment, layer 20 can be thicker,for instance, layer 20 can be a free-standing substrate layer 20 such asmay be placed adjacent to metal film 11 following independent formationof layer 20. For instance, in one embodiment, layer 20 can give supportand definition to film 11.

In general, the wavelength of light passing through a dielectricmaterial will be effected by the refractive index of the material in aknow manner, such that $\lambda_{eff} = \frac{\lambda_{o}}{\eta_{eff}}$

where: λ_(eff) is the wavelength of light in the dielectric

-   -   λ_(o) is the freespace wavelength of the light and    -   η_(eff) is the effective index of refraction of the material.

When considering nonlinear optical materials, the effective refractiveindex of the material can vary depending upon some exterior propertyacting upon the material. For instance, when considering a nonlinearmaterial in which the index of refraction can vary depending on theintensity of the light directed at the material, the wavelength of lightthrough the material can be described as:$\lambda_{eff} = \frac{\lambda_{o}}{\eta_{0} + {\eta_{2}I}}$where η₀ is the nominal refractive index of the material,

-   -   η₂ is the nonlinear refractive index,    -   (η₂I) is the change to the refractive index to the material        under light of intensity I, and    -   η₀+η₂I is the intensity dependant refractive index        Thus, as the intensity I of light directed at the material        increases, the effect on the refractive index of the material,        and hence the wavelength of light through the material, will        also change.

As previously mentioned, in one embodiment, materials for the nonlinearoptical layer substantially adjacent the metallic film can include thosehaving large third order susceptibilities. The nonlinear refractiveindex, η₂, is related to the third order susceptibility, χ⁽³⁾, by:η₂=[χ⁽³⁾×3/(4η₀)]In one embodiment, the nonlinear optical material of the presentinvention can be one in which the nonlinear refractive index, η₂, isgreater than about 10⁻¹⁶ cm²/W.

In this embodiment, the nature and characteristics of the incident lightto the device can determine the transmittivity of the optical devices ofthe present invention.

For instance, in the embodiment shown in FIG. 1, incident zero-orderlight including light at a wavelength λ_(o) is directed at device 10including metal film 11 and layer 20. Layer 20 includes a nonlinearoptical material in which the effective wavelength of light through thematerial is dependent upon the intensity of the incident light, i.e.,$\eta_{eff} = {{\eta_{0} + {\eta_{2}I\quad{and}\quad\lambda_{eff}}} = \frac{\lambda_{o}}{\eta_{eff}}}$

In addition, metal film 11 has been formed to include an array ofapertures 12 of a diameter d set at a periodic spacing p so as to giverise to SPP that can resonantly couple with light at a desiredwavelength λ_(eff). Thus, when the incident light is at an intensity Iso as to yield refractive index λ_(eff) from layer 20, the light canresonantly couple to the plasmons and the transmittivity of the light atλ_(eff) can be greatly enhanced. Thus, the disclosed devices can beself-regulating in that an inherent characteristic of the incident lightcan control the transmittivity of the light through the device.

Beneficially, the resonant coupling of light with plasmons can occur oneither side of the device and the transmittivity of the device can stillbe enhanced. For example, in the embodiment illustrated in FIG. 2, thelayer 20 can be substantially adjacent to the metal film 11 on the side14 of the device, which, in this embodiment, is the side of the devicethat can emit light. This is opposite to the embodiment illustrated inFIG. 1, in which the side 14 is receiving the incident light. In theembodiment illustrated in FIG. 2, the incident light to the device atside 16, including zero-order light at a wavelength λ₀, can still have acertain amount of coupling to the plasmons, though not necessarilyresonant coupling, and can describe a small amount of transmittivity.The photons reradiated by the plasmons from the film 11 in FIG. 2 willstill describe light at a wavelength λ_(eff) however, due to thenonlinear optical dielectric layer 20 substantially adjacent to themetal film on this side of the film, and this light, at λ_(eff) canresonantly couple to the plasmons, with the net effect of thetransmittivity of the light incident to side 16 of device 10 in FIG. 2being the same as that for the embodiment illustrated in FIG. 1, whenthe light was incident to side 14 of the device. In other words, whenresonant coupling between plasmons and photons occurs at any interfaceof a metallic film with an adjacent dielectric material, thetransmittivity of the light through the device at that wavelength can besubstantially enhanced.

There are many applications for the disclosed optical devices. Forinstance, the disclosed devices can be utilized in applications allowingpreferential transmission of light of a particular nature. For example,in one embodiment, the devices can be utilized for enhancing thetransmission of low intensity light, while limiting the transmission ofhigh intensity light. This may find application, for instance, inlong-range sensing applications such as astronomical observations. Inanother embodiment, the devices can be utilized in eye protectiondevices, preventing the transmission of possibly harmful high intensitylight. In this particular embodiment, the devices can be designed suchthat resonant coupling between the incident light and the plasmonsoccurs only at low intensities, i.e., the wavelength at which light canresonantly couple to the plasmons, λ_(c), can correspond to thewavelength obtained from the substantially adjacent layer 20 a lowintensity light, or${\lambda_{c} = {\lambda_{eff} = \frac{\lambda_{o}}{\eta_{0} + {\eta_{2}I}}}},{{when}\quad\eta_{2}{\left. I \right.\sim 0}}$In this embodiment, as η₂I, the change to the refractive index to thematerial under light of intensity I, increases with increasingintensity, the wavelength of light through the layer 20, λ_(eff), willshift away from the resonant coupling wavelength, λ_(c), and thetransmittivity of the device can decrease. Thus, in this particularembodiment, the device can efficiently transmit light of low intensity,but can limit transmission of light at higher intensities.

In other embodiments, the devices of the present invention can findapplication as remote, self-regulating switches. For example, thedevices can be included in switching apparatuses designed to activate atonly specific intensities of incident light, which can be natural light(e.g., sunlight, moonlight, starlight) or man-made light.

As is generally known, the intensity of the light incident upon asurface can depend not only on the nature of the light itself, but alsoupon the angle of incidence of the light. As such, in those embodimentswherein the refractive index of an adjacent layer varies according tothe intensity of the incident light, the self-regulated control of thedisclosed devices can optionally depend upon the incident angle of thelight. For example, in one particular embodiment of the presentinvention, the disclosed devices can operate in an outdoor environment.In this embodiment, the devices can be designed so as to achieve maximumtransmission at a light intensity that can be expected at a specificangle of incident sunlight, i.e., at a specific time of day. Similarly,the disclosed devices can operate in an environment in which the devicecan rotate under a light source. In this embodiment, the device can bedesigned so as to achieve maximum transmission of light at a specificangle of rotation that describes the target angle of incidence,providing incident light at the target light intensity to the device.

The disclosed devices can optionally include additional layers andmaterials in conjunction with the metal films and the adjacent layer.For example, referring to FIG. 3, in one embodiment, the discloseddevices can include an additional layer 22 adjacent to the layer 20.Additional layer 22 can include a single layer of a material or caninclude more than one layer of homogeneous or heterogeneous materials.For instance, layer 22 can be a flexible layer or a rigid layer, asdesired. In one embodiment, layer 22 can include a transparent materialsuch as can offer additional mechanical supporting or connectingcapabilities to the device, for example, a polymer, a sapphire, a glass,or a quartz material.

In another embodiment, layer 22 can provide an additional controlmechanism to the device. For example, in one embodiment, layer 22 caninclude a material having a selectively variable index of refractionthat can be controlled according to an applied electrical field. Forexample, layer 22 can include an electro-optic material, a liquidcrystal material, or a semiconductor layer. For instance, layer 22 caninclude selectively variable refractive index materials such as thosedisclosed by Kim, et al., previously incorporated by reference. In thisparticular embodiment, the device can include a transparent conductivelayer 23 at the surface of layer 22, as shown. During use, the metalfilm 11 and the transparent conductive layer 23 can function aselectrodes to create an electric field that can be utilized to controlthe index of refraction of the layer 22. The transparent conductivelayer can be, for example, an indium tin oxide layer, as is generallyknown in the art.

Yet another embodiment of the present invention is illustrated in FIG.4. In this embodiment, metal film 11 can be sandwiched between layer 20and layer 24. Layer 24 can include a single layer of a material or caninclude more than one layer of homogeneous or heterogeneous materials,as described above for layer 22. For example, layer 24 can include atransparent rigid or flexible substrate such as a polymer, quartz,glass, or sapphire substrate that can protect or otherwise mechanicallyenhance the device. Optionally, layer 24 can provide additionaltransmittivity control to the device. For example, layer 24 can includea non-linear optical material the same or different as that of layer 20.Additionally, layer 24 can include a material having a selectivelyvariable index of refraction that can be controlled according to anapplied electrical field, as described above for layer 22.

In another embodiment, the invention is directed to films that arepolarization-sensitive. According to this embodiment, the film caninclude an array of apertures that are anisotropic, that is, aperturesthat have an aspect ratio greater than one. According to the presentdisclosure, aspect ratio is herein defined as the ratio of the length ofthe major axis of the aperture to that of the minor axis. Accordingly,high aspect ratio or anisotropic apertures includes, for instance,rectangles, ellipses, and the like. In one embodiment, a metal film ofthe present invention can include a periodic array of apertures, each ofwhich having an aspect ratio greater than about two. For example,rectangular or elliptical apertures having a major axis of up to about500 nm and a minor axis less than the major axis can be formed. In oneparticular embodiment, a regular array of anisotropic apertures can beformed on a film each of which having a major axis of about 220 nm and aminor axis less than 220 nm, for example between about 60 nm and about200 nm.

FIG. 5 illustrates one embodiment of a film including a single arrayformed of multiple high aspect ratio apertures 12. It has beendiscovered that films including such high aspect ratio apertures cantransmit polarized light when the electric field (E field) of linearlypolarized incident light is normal to the major axis of the apertures12, with decreasing transmission as the electric field rotates away fromnormal. As shown in FIG. 5, when the E field is normal to the major axisof the apertures 12, that is, in the [0,1] direction, transmissionmaximum occurs, however when the E field of the incident polarized lightis rotated by 90°, such that it is parallel to the major axis, in the[1,0] direction, little or no transmission will occur. In addition, ithas been found that the higher the aspect ratio, the less thetransmittivity through the film when the E field is parallel to themajor axis. That is, as the aspect ratio approaches unity (a square orcircular aperture) the transmission characteristics of the film becomeless selective as to the polarization characteristics of the incidentlight, while at a higher aspect ratio, for example a ratio of about 2:1or greater, essentially no polarized light will be transmitted when theelectric field of the incident light is parallel to the major axis ofthe high aspect ratio apertures.

According to this embodiment, the disclosed films can be polarizingfilms. Accordingly, in one embodiment of the present invention, incidentpolarized light may be selectively transmitted by the disclosed devicesby merely changing the orientation of the incident polarized light withrespect to the film.

In another embodiment, non-polarized light can be incident upon a filmof the present invention including an array of high aspect ratioapertures, and the film can act as a polarizer. In particular, theunpolarized incident light can be transmitted as linearly polarizedlight due to the selective transmission of the array of high aspectratio apertures.

In all other ways, films of this particular embodiment, that is, thoseincluding arrays of high aspect ratio apertures, appear to behave thesame as films including arrays of apertures having an aspect ratio ofone, i.e., square or circular apertures. For example, the particularwavelength of the polarized light that can couple with SPP and beefficiently transmitted through the anisotropic can vary depending uponthe length of the major axis of the apertures as well as on theperiodicity of the array and the thickness of the film, as is generallyknown in the art and discussed previously.

In one embodiment, a film including at least two different overlyingarrays, can be utilized to pattern two separate color transparentstructures in the same region of a perforated film. In particular, oneof the arrays can be an anisotropic polarizing array, as discussedabove. According to this embodiment, a single film can be patterned withtwo or more different overlying arrays of apertures. For example, in oneembodiment, two arrays of anisotropic apertures having differentperiodicities a and b can be formed on a single metal film with themajor axes of the apertures of the two arrays normal to one another.According to this embodiment, when the E field of the incident polarizedlight is normal to the major axis of apertures having a periodicity a,the emission can have the corresponding peaks at A nm. When the E fieldof the linearly polarized light is rotated 90° and is now normal to theapertures with a periodicity b, the transmission peak can shift to B nm.In this way, one region of a film can:transmit multiple colors. Thisdevice can have applications, for instance, in color-shifting pixeltechnology and can significantly improve the resolution of opticaldisplays.

FIG. 6 illustrates one example of this embodiment. FIG. 6A illustrates afilm including a first array of anisotropic apertures, designed in thisparticular embodiment to emit 650 nm linearly polarized light in the[0,1] direction, as shown on the coordinate in the Figure. FIG. 6Billustrates a second film including a second array of anisotropicapertures, designed to emit 550 nm linearly polarized light in the [1,0]direction, as shown. FIG. 6C illustrates a single film including botharrays, overlying each other. The film of FIG. 6C can thus emit both 650nm light and 550 nm light, but the emission characteristics can dependupon the electric field of the incident light. In particular, the filmcan emit 650 nm light when the incident light is polarized in the [0,1]direction and can emit 550 nm light when the incident light is polarizedin the [1,0] direction.

In another embodiment, one of the overlapping arrays can includeanisotropic geometry and another array can include isotropic geometry.In this embodiment, the two arrays have been shown to interact with oneanother, with the result being the suppression of the modes that areinterrupted. This is in contrast to films including overlying polarizingapertures, in which each structure appears to be unaffected by theother. According to this embodiment, the structure of the array can bedesigned to modulate the emission for a specific color. Such structurescan be used, for example, to create color-shifting pixels.

It will be appreciated that the foregoing examples, given for purposesof illustration, are not to be construed as limiting the scope of thisinvention. Although only a few exemplary embodiments of this inventionhave been described in detail above, those skilled in the art willreadily appreciate that many modifications are possible in the exemplaryembodiments without materially departing from the novel teachings andadvantages of this invention. Accordingly, all such modifications areintended to be included within the scope of this invention that isdefined in the following claims and all equivalents thereto. Further, itis recognized that many embodiments may be conceived that do not achieveall of the advantages of some embodiments, yet the absence of aparticular advantage shall not be construed to necessarily mean thatsuch an embodiment is outside the scope of the present invention.

1. An optical device comprising: a film defining at least one aperturetherethrough having a maximum cross-sectional dimension less than thewavelength of light, the film defining a periodic array of structuralelements at a surface of the film, wherein surface plasmon polaritonsarise on the film when light strikes the surface of the film; a firstlayer substantially adjacent to the film, wherein the first layercomprises a material having a selectively variable index of refraction,the refractive index of the first layer varying in response to acharacteristic of the light incident on the optical device.
 2. Theoptical device of claim 1, wherein the film comprises a metal or aheavily doped semi-conductor.
 3. The optical device of claim 1, whereinthe film comprises a material selected from the group consisting ofsilver, aluminum, chromium, gold, titanium, or alloys thereof.
 4. Theoptical device of claim 1, wherein the periodic array of structuralelements is a periodic array of apertures through the film.
 5. Theoptical device of claim 4, wherein the apertures have a maximumcross-sectional dimension of between about 50 and about 500 nanometers.6. The optical device of claim 4, wherein the apertures are of unityaspect ratio.
 7. The optical device of claim 1, wherein the periodicarray of structural elements is a periodic array of superficial featuresdefined by the surface of the film.
 8. The optical device of claim 1,wherein the first layer comprises a nonlinear optical material, whereinthe refractive index of the non-linear optical material varies accordingto the intensity of light incident on the supporting substrate.
 9. Theoptical device of claim 8, wherein the first layer comprises a materialpossessing a nonlinear refractive index greater than about 10⁻¹⁶ cm²/W.10. The optical device of claim 9, wherein the first layer comprises amaterial selected from the group consisting of chlcogenide glasscontaining arsenic, sulfur, selenium, or germanium; oxide glasscontaining silicon, germanium or lead; semiconducting crystalscontaining silicon, germanium, zinc, sulfur, selenium, cadmium, lead, ortellurium; and polymers containing nonlinear chromophores.
 11. Theoptical device of claim 1, wherein the periodicity of the array ofstructural elements is between about 1.414·_(p) and about 2 micrometers.12. The optical device of claim 1, further comprising an additionallayer.
 13. The optical device of claim 12, wherein the additional layeris substantially adjacent to the first layer such that the first layeris between the film and the additional layer.
 14. The optical device ofclaim 12, wherein the additional layer is substantially adjacent to thefilm such that the film is-between the first layer and the additionallayer.
 15. The optical device of claim 12, wherein the additional layercomprises a material having a selectively variable refractive index. 16.The optical device of claim 15, wherein the refractive index of theadditional layer varies in response to a characteristic of the lightincident to the first layer.
 17. The optical device of claim 15, whereinthe refractive index of the additional layer varies in response to anexternally applied electric field established across the additionallayer.
 18. The optical device of claim 1, wherein the device is anoptical switch, an optical filter, an optical modulator, or an opticallimiter.
 19. The optical device of claim 1, wherein the refractive indexof the first layer material varies in response to the angle of incidenceof the light incident to the optical device.
 20. A self-regulatingoptical device, comprising: a metal film defining a periodic array ofapertures of a diameter less than the wavelength of light, whereinsurface plasmon polaritons arise on the film when light strikes asurface of the metal film; a first layer substantially adjacent to themetal film, the first layer comprising a material having a selectivelyvariable refractive index, the refractive index of the material varyingin response to the intensity of light incident to the optical device;and wherein the incident light couples to the surface plasmon polaritonsat a predetermined incident light intensity.
 21. The optical device ofclaim 20, wherein the incident light resonantly couples to the series ofsurface plasmons.
 22. The optical device of claim 21, wherein theincident light resonantly couples to the series of surface plasmons atan intensity between about 10² and about 10¹⁵ photons of green light persquare centimeter per second.
 23. The optical device of claim 20,wherein the metal film comprises a metal selected from the groupconsisting of silver, aluminum, chromium, gold, titanium, or alloysthereof.
 24. The optical device of claim 20, wherein the apertures havea maximum cross-sectional dimension of between about 50 and about 500nanometers.
 25. The optical device of claim 20, wherein the first layercomprises a material possessing a nonlinear refractive index greaterthan about 10⁻¹⁶ cm²/W.
 26. The optical device of claim 20, wherein theperiodicity of the array of structural elements is between 300 and 2000nanometers.
 27. The optical device of claim 20, further comprising anadditional layer.
 28. The optical device of claim 27, wherein theadditional layer comprises a material having a selectively variablerefractive index.
 29. An optical device comprising: a film defining afirst periodic array of apertures therethrough, each aperture of thefirst array having an aspect ratio greater than one, wherein the majoraxis of each aperture is less than the wavelength of light, whereinsurface plasmon polaritons arise on the film when light strikes thesurface of the film; and the film further defining a second, differentperiodic array of apertures therethrough, each aperture of the secondarray having a maximum cross-sectional dimension less than thewavelength of light, wherein the first and second periodic arraysoverlay each other on the film.
 30. The optical device of claim 29,wherein the apertures of the first periodic array each have an aspectratio greater than about
 2. 31. The optical device of claim 29, whereinthe apertures of the second periodic array are of unity aspect ratio.32. The optical device of claim 29, wherein the apertures of the secondperiodic array are anisotropic apertures having an aspect ratio greaterthan
 1. 33. The optical device of claim 32, wherein the major axis ofthe apertures of the first array is normal to the major axis of theapertures of the second array.
 34. The optical device of claim 29,further comprising a layer substantially adjacent to the film, whereinthe layer comprises a material having a selectively variable index ofrefraction.
 35. A method of modulating light comprising: providing afilm, the film defining a periodic array of anisotropic aperturestherethrough, the apertures extending from a first surface of the filmto a second, opposite surface of the film, each aperture having a majoraxis and a minor axis, wherein surface plasmon polaritons arise on thefilm when light strikes the surface of the film; providing incidentpolarized light to the first surface of the film; and selectivelyemitting light from the film, the light emission depending upon thedirection of the electric field of the incident polarized light.
 36. Themethod of claim 35, wherein the incident polarized light has an electricfield normal to the major axis of the apertures, wherein the incidentpolarized light is reradiated from the opposite surface of the film. 37.The method of claim 35, wherein the incident polarized light has anelectric field parallel to the major axis of the apertures, wherein theincident polarized light is not reradiated from the opposite surface ofthe film.
 38. A method of modulating light comprising: providingincident light at a first wavelength to an optical device comprising alayer, the layer comprising a material having a selectively variablerefractive index; varying the initial refractive index of the layer to asecond refractive index in response to a characteristic of the incidentlight; altering the first wavelength to a second wavelength according tothe second refractive index of the layer; providing a film, the filmdefining a periodic array of structural elements at a surface of thefilm, wherein surface plasmon polaritons arise on the film when theincident light strikes the surface of the film, the film defining atleast one aperture therethrough having a maximum cross-sectionaldimension less than both the first and the second wavelengths, whereinthe film is substantially adjacent to the layer; and emitting light atthe second wavelength from the optical device.
 39. The method of claim38, wherein the periodic array of structural elements is a periodicarray of apertures through the film.
 40. The method of claim 39, whereinthe apertures have an anisotropic aspect ratio.
 41. The method of claim40, wherein the emitted light is polarized light.
 42. The method ofclaim 38, wherein characteristic of the incident light is the intensityof the incident light.